Estradiol affects many central neural functions but the mechanisms are not well understood. Gonadotropin- releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction. GnRH neurons are output neurons of the central nervous system and the effects of estradiol on fertility in the whole animal are well understood. By determining the mechanisms by which estradiol alters neural networks using GnRH neurons as a model, we will provide information vital for human health and designing new strategies for fertility and contraception. The work will also be relevant to estradiol action on more complex neural systems that are not as approachable for study and for which the output is not well defined, such as those governing cognition and memory formation. State-of-the-art electrophysiological approaches will be used to investigate the neurobiological mechanisms underlying estradiol feedback on GnRH neurons. The increase in estradiol that occurs during the female reproductive cycle has a biphasic effect on GnRH release. Initially estradiol inhibits GnRH release via negative feedback. With sustained exposure to elevated estradiol, however, the response to this steroid switches from negative to positive. Positive feedback induces a surge in GnRH release that is the neural prerequisite for ovulation. Using a mouse model that generates daily transitions between negative and positive feedback upon exposure to constant physiological estradiol levels, three specific aims concerning the actions of estradiol will be addressed. Comparisons will be made with data from animals in which the feedback loop was opened by castration. In Aim 1, the effects of estradiol on fast synaptic transmission to GnRH neurons will be studied, including activity of afferent neurons and responsiveness of GnRH neurons. In Aim 2, the effects of estradiol on intrinsic conductances of GnRH neurons will be explored, including whether estradiol-induced changes are due to changes in phosphorylation of ion channels. In Aim 3, the effects of three selected neuromodulators hypothesized to mediate estradiol feedback will be examined. Together these studies will provide insight into the direct and transsynaptic mechanisms utilized by estradiol to bring about negative and positive feedback regulation of GnRH release.